1. Field of the Invention
The present invention relates to a method of preventing the corrosion of tungsten plugs during semiconductor device fabrication processes.
2. Background of the Invention
As semiconductor device dimensions shrink, it has become necessary to conserve area on the semiconductor wafer surface, especially during the real-estate consuming process of fabricating several stacked layers of interconnect wires (metallization levels). In many ways, device density on a chip is now interconnect-limited. In previous years, when device dimensions (and hence interconnect wires) were larger, wires usually completely covered the underlying tungsten (W) plugs (referred to hereinafter as either a xe2x80x9ctungsten plugxe2x80x9d or a xe2x80x9cW plugxe2x80x9d) at their contact point, and corrosion of the tungsten was a concern only when misalignment of an optical projection lithography stepper resulted in a patterned wire layer not aligned squarely over the top of a given W plug. Now, because of ever-shrinking device dimensions (and the need for tighter control over interconnect wire size), wire layers are often purposefully formed so that they do not completely cover an underlying W plug. As such, the corrosion of W plugs and the overlying wires are still a concern in the fabrication process.
Generally, after forming a W plug, a wire is formed to couple with the W plug. This wire material has typically been Al-(0.5 wt %-1.5 wt %)Cu. Although in the last few years, as device dimensions shrink below the 0.18 xcexcm design rule, Cu wiring materials have been replacing Al-based alloys as the multi-level interconnect material of choice.
FIG. 1 is a cross-sectional view (not to scale) of a partially-processed semiconductor device according to the prior art, showing a substrate 10, interlevel dielectric material 11, a xe2x80x9cglue layerxe2x80x9d or adhesion layer 12 comprising TiN, TiW, or other barrier material, tungsten (W) plug 13, polymeric resist (photoresist) material 14 for pattern definition, metal wire 15, and a polymeric residue (etch byproduct) 17. The interconnect wire 15, as shown, does not completely cover the underlying W plug 13. This patterned wire 15 may be accidentally misaligned over the W plug 13, or purposefully formed to cover only part of the W plug 13 (to conserve chip area). It will be understood by those skilled in the art that the cross-sectional views presented in all the drawings omit several known components of a semiconductor device/integrated circuit (IC) for the purposes of clarity.
FIG. 2 is a cross-sectional view (not to scale) of a partially-processed semiconductor device according to FIG. 1, showing some polymeric material 14 (FIG. 1) (which has been generated from dry etching) remaining as polymeric residue 17 after a dry etching step. The exposed portion 16 of W plug 13 is evident adjacent to the polymeric residue 17. While performing this oxygen plasma etching process to ash the photoresist 14 and pattern interconnect wire 15, some residual photoresist or other polymeric residue 17 is inevitably left behind and remains adhered to the side surfaces of patterned wire 15 and photoresist 14. This polymeric residue 17 must be removed before the fabrication process can continue. The oxygen plasma etching step described above is typically performed to remove photoresist material 14, followed by a wet-cleaning process utilizing a stripping solution (e.g. EKC-265(trademark) from EKC Technology Inc. of Hayward, Calif.) with a conventional pH of about 10-12 to remove polymeric residue 17.
In a case, such as that described above, where interconnect wire 15 is misaligned over W plug 13, or in a case where interconnect wire 15 is purposefully patterned to incompletely cover W plug 13, a portion 16 of the W plug 13 surface is exposed during subsequent processing steps.
As shown in FIG. 3 (a cross-sectional view (not to scale) of a partially-processed semiconductor device according to the prior art), W plug 13 corrosion occurs during the standard wet-stripping process used to remove remaining polymeric residue 17 from wire 15.
While the wet-cleaning process is performed with stripping solution 19 to remove polymeric residue 17 on the sides of patterned metal wire 15, the exposed portion of W plug 13 is corroded by stripping solution 19 and a hole 18 is formed in W plug 13. This tungsten corrosion is caused by charges (xe2x80x9cqxe2x80x9d) 20 accumulated on the surface of interconnect wire 15 either while performing the interconnect wire 15 etching process to pattern said wire 15, or when oxygen plasma ashing the photoresist 14. The charged wire and W plug 13 exhibit a large electrical potential between them (the two dissimilar metals have different electrochemical potentials and essentially form a galvanic couple). As a result, the exposed tungsten is oxidized to an ionic state 21 (WO4xe2x88x922, for example) by the stripping solution 19, which has a pH value conventionally from 10 to 12. The exposed tungsten is stripped from the surface of W plug 13 during this wet-cleaning process, resulting in the abovementioned hole 18.
FIG. 4 is a cross-sectional view (not to scale) of a partially-processed semiconductor device according to the prior art, showing the W plug 13 corrosion hole 18 after the standard wet-stripping process discussed above and illustrated in FIG. 3. Because the area of contact between W plug 13 and patterned metal wire 15 is reduced by corrosion of W plug 13, electrical resistance is increased in the wiring lines and this inevitably leads to catastrophic failure of the integrated circuit (IC).
The conventional prior art solution to the abovementioned corrosion problem is to dip a partially-processed substrate into a neutral ionic solution (e.g. electrolyte) or deionized water for several hours, and then to perform the wet cleaning process with the stripping solution. By dipping the substrate into a neutral ionic solution, the charges (q) accumulated on the wire surface are effectively discharged. Nevertheless, while this protects W plug 13 from electro-galvanic corrosion, interconnect wire 15 on top of W plug 13 is instead corroded by the neutral ionic solution (i.e. the metal in wiring 15 reacts with the salts/electrolytes in the neutral ionic solution). As wire dimensions continue to shrink, preventing corrosion of the wire is important along with preventing corrosion of W plug 13, for reasons already mentioned. Furthermore, after the substrate dipping step, a separate rinse-dry step is required for this method to remove any residual solution (i.e. salts/electrolytes) from the substrate.
Therefore, the present invention provides a method of preventing W plugs and metal wires from corrosion during semiconductor device fabrication. According to the present invention, the method comprises steps of providing a W plug formed in a substrate and coupled with a wire formed on the substrate. The substrate is then dipped into a non-ionic benign solvent, such as pure isopropyl alcohol (IPA) or pure N-methyl pyrrolidone (NMP), and a rinse process is performed to clean a surface of the wire. Then, the substrate is spin-dried and a conventional wet-stripping process is performed.
This invention uses the IPA or NMP solvents to discharge the electrical charges (q) accumulated on the wire, thereby preventing the W plugs from electro-galvanic corrosion. Once the charge accumulated on the wire is discharged, there is no longer a large electrical potential generated between the W plug and the wire during the subsequent wet-cleaning step. As a result, the exposed tungsten is unlikely to oxidize and the W plugs are protected from corrosion. Furthermore, the IPA and NMP solvents do not chemically attack the interconnect wire patterned over the W plug, and consequently wire corrosion is also prevented.
Since IPA and NMP solvents are commonly used in an inter-medium rinse process performed after oxygen plasma etching and wet cleaning (e.g. with EKC-265(trademark)) and before a Quick Dump Rinse (QDR) process, pure IPA and pure NMP solvents are readily available from a general fabrication facility. In addition, the duration of dipping the substrate into the pure IPA or pure NMP solvent is up to only a few minutes, which is a substantially shorter duration than the prior art neutral ionic solution dipping process. Therefore, the process according to an aspect of the present invention simplifies the processing sequence of the semiconductor metallization layers and is cost effective. Efficiency can be improved and processing time reduced without the need to change fabrication equipment or to introduce additional potential contaminants into the production line.
Furthermore, using the IPA or NMP solvents to discharge the electrical charge accumulation on the wire prevents the W plugs from electro-galvanic corrosion, since the IPA or NMP solvents are chemically compatible with wet cleaning processes (e.g. with EKC-265(trademark)). A direct integrated IPA or NMP discharge step before the wet cleaning is possible, which makes the process simple and more cost effective.
The present invention provides a method for preventing the corrosion of W plugs and patterned metal layers when polymeric residue is removed with a wet-stripping solution during manufacture of semiconductor devices.
In accordance with an embodiment of the present invention, a method of preventing tungsten plugs from corrosion comprises providing a tungsten plug formed in a substrate and coupled with a wire formed on the substrate, dipping the substrate into a non-ionic benign solvent, performing a rinsing process to clean a surface of the wire, and performing a spin-drying step. The substrate is dipped in the non-ionic benign solvent, such as IPA or NMP, before the standard wet stripping process is performed. The non-ionic benign solvent substantially discharges a net charge accumulated on a surface of the wire during fabrication of a semiconductor device. The substrate dipping time can vary from about 1 second to 5 minutes.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention.